A photorefractive device is a non-linear optical device made in accordance with the photorefractive effect of a crystal(s), which can be a one piece crystal processed according to a certain manner or an optoelectronic system including at least a one piece crystal. The principle and structure of the photorefractive device can be found in various literature, books and proceedings of related symposia. For example, "The photorefractive materials and their application" edited by P. Gunter et al, Spinger-Verlay, published in 1988-1989, and the proceedings on the first three international conferences "On Photorefractive Material, Effect and Devices" in 1988, 1990 and 1991 respectively. This type of device has significant applications in interferometry, phase locking of laser diodes, holography storage, optical image processing, amplification of weak optical image signals, massively interconnected neural-network architecture in optical computing and other new technologies.
Ferroelectric oxide crystals are the most important kind of photorefractive materials. Among them, the tetragonal phase barium titanate crystals are the crystals which have been studied most. The reasons are as follows: firstly, barium titanate crystal possesses excellent photorefractive property and large photorefractive effect; secondly, barium titanate crystal is a single compound. Therefore, it has good optical quality, which is important in the application of optical image processing, amplification of weak optical image signals, holography, massively interconnection of neural-networks, etc. However, undoped barium titanate crystal still has some deficiency, such as longer self-pumped phase conjugation buildup time, longer two wave coupling response time, more strict requirements on the angle and point of incidence. Therefore, it is not easy to adjust them to their optimum values.
The photorefractive property can be improved by doping a little amount of some elements into a pure barium titanate crystal as impurities. It has been reported in the literature that the addition of Fe, Co, Ni, Nb as the impurity elements into a pure barium titanate crystal can change the photorefractive effects of barium titanate crystals. However, the deficiency of undoped barium titanate crystals mentioned above has still not been totally overcome.